The present invention relates to a sizing, agent for carbon fibers and more particularly to a sizing agent for carbon fibers which contains an isocyanate regenerating compound. According to the present invention, there are provided sizing agents which can improve various properties of carbon fiber reinforced composite materials and carbon fibers which are improved in bonding to resins.
Carbon fibers are combined with resins and the composites are widely used in various fields such as aircrafts, automobiles, ships, sports goods and the like.
Normally, these carbon fibers are subjected to surface treatment in order to improve adhesion to matrix resins to afford excellent performances as composite materials.
For the surface treatment, generally, the surface of carbon fibers is oxidized and then sized with an epoxy resin (Japanese Patent Kokai No. 61-252371). It is further proposed to use as a sizing agent a mixture of epoxy resin and polyurethane resin (Japanese Patent Kokai No. 62-110984) and a polyurethane resin (Japanese Patent Kokai No. 58-126375).
In order to make the best use of the excellent properties of carbon fibers in composite with resins, it is necessary to enhance the bonding between carbon fiber and matrix resin. For this purpose, various sizing agents have been used and it is said that carbon fiber coated with polyurethane resin is effective. (Japanese Patent Kokai No. 58-126375). However, this method is still insufficient in utilization of performance of carbon fiber.